The invention is based on a hydraulic dual-circuit brake system with an anti-skid system (ABS) and traction control (ASR) for motor vehicles, as defined hereinafter. Such a brake system is known from German Patent 38 00 854 now U.S. Pat. No. 4,846,532. Here, the pump feed apparatus for ASR operation is embodied by a vacuum cell and a switch valve, which in the ASR mode connects the vacuum cell pressure chamber to the inlet of the return pump element that is associated with the brake circuit of the driven wheels of the rear axle. By means of the charge pressure generated by the vacuum cell in the brake-fluid-filled pressure chamber, brake fluid is fed into the pump element, which in turn applies an approximately constant brake supply pressure to the control valves associated with the driven wheels. The brake pressure required in the wheel brake cylinder of a spinning wheel to brake it is adjusted by switching the associated control valve.
European Patent 0 062 246 discloses a dual-circuit brake system with front-axle/rear-axle brake circuit distribution, which has an adaptor to distribute the brake force, fed by the master brake cylinder, to the wheels of the front and rear axles. The adaptor has a hydraulic actuating device that is controlled by a microprocessor such that upon braking, the brake pressure in the brake circuit of the rear axle is adjusted as a function of the brake pressure in the front axle brake circuit, such that the brake pressure in the rear axle brake circuit is always somewhat lower than a value obtained from a mathematical expression stored in the microprocessor. By means of this adaptor, the rear axle braking can be selected freely, in accordance with an ideal distribution between front and rear axle brake pressure, so that in every phase of driving, any unstable driving situations, such as those resulting from overbraking of the rear axle, are avoided.